Multifunctional autonomically healing composite material

ABSTRACT

A composite material, contains a polymer, a polymerizer, a corresponding catalyst for the polymerizer, and a plurality of capsules. The polymerizer is in the capsules. The composite material is self-healing.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The subject matter of this application may in part have been funded bythe Air Force (AFOSR Grant no. F49620-00-1-0094/White). The governmentmay have certain rights in this invention.

BACKGROUND

The present invention relates to self-healing composite materials.

Thermosetting polymers, used in a wide variety of applications rangingfrom microelectronics to composite airplane wings, are susceptible todamage in the form of cracking. Often these cracks form deep within thestructure where detection is difficult and repair is virtuallyimpossible. In fiber reinforced polymer composites, cracking in the formof fiber-matrix interfacial debonding, ply delamination, and simplematrix cracking leads to degradation. In microelectronics, polymerencapsulates and polymer matrix composite printed circuit boards sufferfrom similar forms of damage, but in addition to mechanical failure,cracks cause electrical failure of the component. Microcracking inducedby thermal and mechanical fatigue is a longstanding problem in polymeradhesives. Regardless of the application, once cracks have formed withinpolymeric materials, the integrity of the structure is significantlycompromised. Typically, previously reported methods of successful crackhealing require some form of manual intervention.

A proposed method of self-healing is described in “Self-HealingComposites Using Embedded Microspheres” D. Jung et al. Composites andFunctionally Graded Materials vol. MD-80, in Proceedings of the ASMEInternational Mechanical Engineering Conference and Exposition, 265-275(1997). The proposed method uses polyoxymethyleneurea (PMU) microspheresto store a crack filling agent to be released into the crack and rebondthe crack faces. The repair mechanism uses naturally occurringfunctional sites in a polyester matrix network to trigger the repairaction. Adding a reactive component to trigger the crack fillersolidification was specifically investigated in the case of embeddedepoxide components and embedded amine groups, and it was found that theamine groups did not retain sufficient activity and was determined to benot feasible. The PMU microcapsules used contained an epoxide monomer.

BRIEF SUMMARY

In a first aspect, the present invention is a composite material,containing: a polymer, a polymerizer, a corresponding catalyst for thepolymerizer, and a plurality of capsules. The polymerizer is in thecapsules.

In a second aspect, the present invention is a composite material,containing: a polymer, a polymerizer, a corresponding activator for thepolymerizer, and a first plurality of capsules. The polymerizer is inthe capsules, and the corresponding activator is not a native activatingmoiety.

In a third aspect, the present invention is a method for making theabove composites, including dispersing the capsules and thecorresponding catalyst or activator into the polymer.

Definitions

A polymerizer is a composition that will form a polymer when it comesinto contact with a corresponding activator for the polymerizer.Examples of polymerizers include monomers of polymers such as styrene,ethylene, (meth)acrylates, and dicyclopentadiene (DCPD); a monomer of amulti-monomer polymer system such as diols, diamines, and epoxide; andprepolymers such as partially polymerized monomers still capable offurther polymerization.

An activator is anything that when contacted or mixed with a polymerizerwill form a polymer. Examples of activators are catalysts, initiators,and native activating moieties. A corresponding activator for apolymerizer is an activator that when contacted or mixed with thatspecific polymerizer will form a polymer.

A catalyst is a compound or moiety that will cause a polymerizablecomposition to polymerize, and is not always consumed each time itcauses polymerization. This is in contrast to initiators and nativeactivating moieties. Examples of catalysts include ring openingpolymerization (ROMP) catalysts such as Grubbs catalyst. A correspondingcatalyst for a polymerizer is a catalyst that when contacted or mixedwith that specific polymerizer will form a polymer.

An initiator is a compound that will cause a polymerizable compositionto polymerize, and is always consumed at the time it causespolymerization. Examples of initiators are peroxides (which will form aradical to cause polymerization of an unsaturated monomer); a monomer ofa multi-monomer polymer system such as diols, diamines, and epoxide; andamines (which will form a polymer with an epoxide). A correspondinginitiator for a polymerizer is an initiator that when contacted or mixedwith that specific polymerizer will form a polymer.

A native activating moiety is a moiety of a polymer that when mixed orcontacted with a polymerizer will form a polymer, and is always consumedat the time it causes polymerization. Examples of a native activatingmoiety is an amine moiety (which will form a polymer with an epoxide).

A compound is a molecule that contains at most 100 repeating units. Thisis in contrast to a polymer, which contains more than 100 repeatingunits.

A capsule is a hollow closed object having an aspect ratio of 1:1 to1:10. The aspect ratio of an object is the ratio of the shortest axis tothe longest axis; these axes need to be perpendicular. A capsule mayhave any shape that falls within this aspect ratio, such as a sphere, atoroid, or an irregular ameboid shape. The surface of a capsule may haveany texture, for example rough or smooth.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood from the following detailed description when considered inconnection with the accompanying drawings in which like referencecharacters designate like or corresponding parts throughout the severalviews and wherein:

FIG. 1 illustrates an embodiment of a self-healing composite; and

FIG. 2 shows crack healing efficiency of the composite materials.

DETAILED DESCRIPTION

Further investigations by the group that published “Self-HealingComposites Using Embedded Microspheres” found that the use of naturalfunctionality was, in fact, not feasible. Approaches that had beenpreviously eliminated were reconsidered, resulting in the discovery thatsystems that do not use a corresponding native activating moiety willallow for a self-healing composite. Preferably, systems that use acatalyst added to the polymer are used. Not only can damage to thecomposite be repaired, but also in some cases the achieved healedstrengths are greater than the strength of the original matrix material.

The present invention includes a composite material, containing capsulesin a polymer. The capsules contain a polymerizer, and the compositematerial includes an activator that is not a corresponding nativeactivating moiety. Preferably, the activator is a corresponding catalystfor the polymerizer. When a crack forms in the composite material, someof the capsules are broken, and the polymerizer moves into the crack,coming into contact with the activator and forming a polymer. Thisrepairs the crack.

The capsules contain a polymerizer. The polymerizer contains apolymerizable compound such as a monomer or prepolymer, and mayoptionally contain other ingredients, such as other monomers and/orprepolymers, stabilizers, solvents, viscosity modifiers such aspolymers, odorants, colorant and dyes, blowing agents, antioxidants, andco-catalysts. Preferably, the polymerizer is a liquid.

The polymer contains both capsules and a corresponding activator for thepolymerizer. Preferably, the activator is a catalyst or an initiator.Examples of polymerizable compounds are cyclic olefins, preferablycontaining 4-50 carbon atoms and optionally containing heteratoms, suchas DCPD, substituted DCPDs, norbornene, substituted norbornene,cyclooctadiene, and substituted cyclooctadiene. Corresponding catalystsfor these are ring opening metathesis polymerization (ROMP) catalystssuch as Schrock catalysts (Bazan, G. C.; Schrock, R. R.; Cho, H.-N.;Gibson, V. C. Macromolecules 24, 4495-4502 (1991)) and Grubbs catalysts(Grubbs, R. H.; Chang, S. Tetrahedron 54, 4413-4450 (1998)).

Another example of polymerizable compounds are lactones such ascaprolactone, and lactams, that when polymerized will form polyestersand nylons, respectively. Corresponding catalysts for these are cyclicester polymerization catalysts and cyclic amide polymerizationcatalysts, such as scandium triflate.

Furthermore, a polymerizer may contain a polymerizable compound and onepart of a two-part catalyst, with a corresponding initiator being theother part of the two-part catalyst. For example, the polymerizablecompound may be a cyclic olefin; one part of a two-part catalyst may bea tungsten compound, such as an organoammonium tungstate, anorganoarsonium tungstate, or an organophosphonium tungstate; or amolybdenum compound, such as organoammonium molybdate, an organoarsoniummolybdate, or an organophosphonium molybdate. The second part of thetwo-part catalyst may be an alkyl aluminum compound, such as analkoxyalkylaluminum halide, an aryloxyalkylaluminum halide, or ametaloxyalkylaluminum halide in which the metal in the compound is tin,lead, or aluminum; or an organic tin compound, such as a tetraalkyltin,a trialkyltin hydride, or a triaryltin hydride.

In another such system, the polymerizable compound may be unsaturatedcompounds such as acrylates; acrylic acids; alkyl acrylates; alkylacrylic acids; styrenes; isoprene; and butadiene. In this case, atomtransfer radical polymerization (ATRP) may be used, with one of the twocomponents being mixed with the polymerizable compound and the otheracting as the initiator: one component being an organohalide such as1-chloro-1-phenylethane, and the other component could be a copper(I)source such as copper(I) bipyridyl complex. Alternatively, one componentcould be a peroxide such as benzoyl peroxide, and the other componentcould be a nitroxo precursor such as2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). These systems aredescribed in Malcolm P. Stevens; Polymer Chemistry: An Introduction, 3rdEdition; New York: Oxford University Press, 1999, p. 184-186.

In another such system, the polymerizable compound may containisocyanate functional groups (—N═C═O) with hydroxyl functional groups(—OH). For this system, the polymerizable material may for example be acompound containing both an isocyanate group and a hydroxyl group, ortwo different compounds, one compound containing at least two isocyanategroups and the other compound containing at least two hydroxyl groups.The reaction between an isocyanate group and a hydroxyl group can form aurethane linkage (—N—C(═O)—O—) between the compounds, possibly releasingcarbon dioxide. This carbon dioxide can provide for the creation ofexpanded polyurethane foam; optionally the polymerizer may contain ablowing agent, for example a volatile liquid such as dichloromethane. Inthis case, condensation polymerization may be used, with one of the twocomponents being mixed with the polymerizable compound and the otheracting as the initiator: for example, one component could be an alkyltincompound such as stannous 2-ethylhexanoate, and the other componentcould be a tertiary amine such as diazabicyclo[2.2.2]octane (DABCO).These systems are described in Malcolm P. Stevens; Polymer Chemistry: AnIntroduction, 3rd Edition; New York: Oxford University Press, 1999, p.378-381.

Optionally, the activator, such as the catalyst or initiator may also bein a separate set of capsules. Furthermore, this separate set ofcapsules may also contain stabilizers, solvents, viscosity modifierssuch as polymers, odorants, colorant and dyes, blowing agents,antioxidants, and co-catalysts. Optionally, a set of capsules may bepresent that contain one or more additional ingredients, such asstabilizers, solvents, viscosity modifiers such as polymers, odorants,colorant and dyes, blowing agents, antioxidants, and co-catalysts.

The capsules contain a polymerizer. Preferably, the capsules have anaverage diameter of 10 nm to 1 mm, more preferably 30-500 μm, mostpreferably to 50-300 μm. The capsules have an aspect ratio of 1:1 to1:10, preferably 1:1 to 1:5, more preferably 1:1 to 1:3, and even morepreferably 1:1 to 1:2, and most preferably 1:1 to 1:1.5.

The wall thickness of the capsules is preferably 100 nm to 3 μm. Theselection of capsule walls thickness depends on the polymer in thecomposite. For example, capsule walls that are too thick will notrupture when a crack approaches, while capsules with very thin wallswill break during processing.

The adhesion between the capsules and the polymer of the compositeinfluences whether the capsules will rupture or debond in the presenceof an approaching crack. To promote the adhesion between the polymer andcapsule wall, various silane coupling agents may be used. Typically,these are compounds of the formula R—SiX₃ Where R is preferably areactive group R¹ separated by a propylene group from silicon, and X isan alkoxy group (preferably methoxy), such as R¹CH₂CH₂CH₂Si(OCH₃)₃.Examples include silane coupling agents available from DOW CORNING (withreactive group following the name in parentheses): Z6020 (Diamino);Z6030 (Methacrylate); Z6032 (Styrylamine Cationic); Z6040 (Epoxy); andZ6075 (Vinyl).

To increase the adhesion between the capsules and a polymer in thecomposite, the capsules may be treated by washing them in a solution ofthe coupling agent. For example, urea-formaldehyde capsules may bewashed in a solution of Silane Z6020 or Z6040 and hexane (1:20 wt.)followed by adding Silane Z6032 to the polymer (1% wt.).

Capsules may be made by a variety of techniques, and from a variety ofmaterials, such as those described in Microencapsulation: Methods andIndustrial Applications Ed. Benita, Simon Marcel Dekker, New York, 1996;Microencapsulation: Processes and Applications Ed. Vandegaer, J. PlenumPress, New York, 1974; and Microcapsule Processing and Technology Kondo,A. Marcel Dekker, New York, 1979. Examples of materials from which thecapsules may be made, and the techniques for making them include:urea-formaldehyde, formed by in situ polymerization; gelatin, formed bycomplex coacervation; polyurea, formed by the reaction of isocyanateswith a diamine or a triamine, depending on the degree of crosslinkingdesired (the extent of crosslinking also determines the brittleness ofthe capsule); and polyamide, formed by the use of a suitable acidchloride and a water soluble triamine.

The polymer may be any polymeric material into which the capsules may bedispersed. Examples include polyamides such as nylons; polyesters suchas poly(ethylene terephthalate) and polycaprolactone; polycarbonates;polyethers such as epoxides; polyimides such as polypyromellitimide (forexample KAPTAN); phenol-formaldehyde resins (for example BAKELITE);amine-formaldehyde resins such as a melamine resin; polysulfones;poly(acrylonitrile-butadiene-styrene) (ABS); polyurethanes; polyolefinssuch as polyethylene, polystyrene, polyacrylonitrile, polyvinyls,polyvinyl chloride, poly(DCPD) and poly(methyl methacrylate);polysilanes such as poly(carborane-siloxane); and polyphosphazenes.

The capsules and activator (such as the catalyst or initiator) may bedispersed into the polymer by forming the polymer around the capsulesand activator, such as by polymerizing monomer to form the polymer withthe capsules and activator mixed into the monomer. Particularly in thecase of catalysts, the catalyst may serve as both a catalyst for thepolymer and as the corresponding activator for the polymerizer in thecapsules. Examples of this system include DCPD as the polymerizer, thepolymer is poly(DPCD), and a Grubbs catalyst serves to form thepoly(DPCD) and acts as the activator for the DCPD in the capsules; andcaprolactone as the polymerizer, the polymer is poly(caprolactone), andscandium triflate acts as the activator for the caprolactone in thecapsules.

Alternatively, the polymer may be first formed, and then the capsulesand activator mixed in. For example, the polymer may be dissolved in asolvent and the capsules and activator mixed into the solution, followedby removal of the solvent. The activator may be coated onto the capsulesprior to dispersing the capsules into the polymer. Furthermore, othercomponents may be added to the polymer, such as fibers, fillers,adhesion modifiers, blowing agents, anti-oxidants, colorants and dyes,and fragrances.

FIG. 1 illustrates an embodiment of a self-healing composite. Anapproaching crack ruptures embedded capsules (referred to asmicrocapsules in the figure) releasing polymerizer (referred to ashealing agent in the figure) into the crack plane through capillaryaction. Polymerization of the healing agent may be triggered by contactwith the activator (here a catalyst), bonding the crack faces. Thedamage-induced triggering mechanism provides site-specific autonomiccontrol of the repair. As shown in FIG. 1, an encapsulated healing agentis embedded in a structural composite matrix containing a catalystcapable of polymerizing the healing agent: (i) cracks form in the matrixwherever damage occurs, (ii) The crack ruptures the microcapsules,releasing the healing agent into the crack plane through capillaryaction, (iii) The healing agent contacts the catalyst triggeringpolymerization that bonds the crack faces closed.

EXAMPLES

The following examples and preparations are provided merely to furtherillustrate the invention. The scope of the invention is not construed asmerely consisting of the following examples.

General Procedure for Preparation of Capsules by In Situ Polymerization

In a 600 mL beaker is dissolved urea (0.11 mol, 7.0 g) followed byresorcinol (0.5 g) and ammonium chloride (0.5 g) in water (150 ml). A 5wt. % solution of ethylene maleic anhydride copolymer (100 mL) is addedto the reaction mixture, and the pH of the reaction mixture is adjustedto 3.5 using 10% NaOH solution. The reaction mixture is agitated at 454rpm, and to the stirred solution is added 60 mL of dicyclopentadiene toachieve an average droplet size of 200 μm. To the agitated emulsion isadded 37% formaldehyde (0.23 mol, 18.91 g) solution, and then thetemperature of the reaction mixture is raised to 50° C. and maintainedfor 2 h. After 2 h, 200 mL of water is added to the reaction mixture.After 4 h, the reaction mixture is cooled to room temperature, andcapsules are separated. The capsule slurry is diluted with an additional200 mL of water and washed with water (3×500 mL). The capsules areisolated by vacuum filtration, and air-dried. Yield: 80%. Average size:220 μm.

Composite Epoxy Specimen Manufacture

The epoxy matrix composite was prepared by mixing 100 parts EPON 828(Shell Chemicals Inc.) epoxide with 12 parts DETA (diethylenetriamine)curing agent (Shell Chemicals Inc.). Composite epoxy specimens wereprepared by mixing 2.5% (by wt.) Grubbs' catalyst and 10% (by wt.)capsules with the resin mixture described above. The resin was thenpoured into silicone rubber molds and cured for 24 h at roomtemperature, followed by postcuring at 40° C. for 24 h.

EXAMPLE

DCPD filled capsules (50-200 μm average diameter) with aurea-formaldehyde shell were prepared using standard microencapsulationtechniques. The capsule shell provides a protective barrier between thecatalyst and DCPD to prevent polymerization during the preparation ofthe composite.

The reaction scheme for the polymerization of DCPD is shown below

To assess the crack healing efficiency of these composite materials,fracture tests were performed using a tapered double-cantilever beam(TDCB) specimen (FIG. 2). Self-healing composite and control sampleswere fabricated. Control samples consisted of: (1) neat epoxy containingno Grubbs' catalyst or capsules, (2) epoxy with Grubbs' catalyst but nocapsules and (3) epoxy with capsules but no catalyst. A sharp pre-crackwas created in the tapered samples by gently tapping a razor blade intoa molded starter notch. Load was applied in a direction perpendicular tothe pre-crack (Mode I) with pin loading grips as shown in FIG. 2. Thevirgin fracture toughness was determined from the critical load topropagate the crack and fail the specimen. After failure, the load wasremoved and the crack allowed to heal at room temperature with no manualintervention. Fracture tests were repeated after 48 hours to quantifythe amount of healing.

A representative load-displacement curve is plotted in FIG. 2demonstrating ca. 75% recovery of the virgin fracture load. In greatcontrast, all three types of control samples showed no healing and wereunable to carry any load upon reloading. A set of four independentlyprepared self-healing composite samples showed an average healingefficiency of 60%. When the healing efficiency is calculated relative tothe critical load for the virgin, neat resin control (upper horizontalline in FIG. 2), a value slightly greater than 100% is achieved. Theaverage critical load for virgin self-healing samples containingcabsules and Grubbs' catalyst was 20% larger than the average value forthe neat epoxy control samples, indicating that the addition of capsulesand catalyst increases the inherent toughness of the epoxy.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1-12. (canceled)
 13. A composite material, comprising: (i) a polymer,(ii) a polymerizer, (iii) a corresponding activator for the polymerizer,and (iv) a first plurality of capsules, wherein the polymerizer is inthe capsules, and the corresponding activator is not a native activatingmoiety. 14-23. (canceled)
 24. A composite material, comprising: (i) apolymer, (ii) a polymerizer, (iii) a corresponding initiator for thepolymerizer, and (iv) a plurality of capsules, wherein the polymerizeris in the capsules.
 25. The composite material of claim 24, wherein thepolymerizer comprises at least one monomer selected from the groupconsisting of cyclic olefins, lactones, lactams, acrylates, acrylicacids, alkyl acrylates, alkyl acrylic acids, styrenes, isoprene andbutadiene.
 26. The composite material of claim 24, wherein thepolymerizer comprises cyclic olefins.
 27. The composite material ofclaim 24, wherein the polymer comprises at least one member selectedfrom the group consisting of polyamides, polyesters, polycarbonates,polyethers, polyimides, phenol-formaldehyde resins, amine-formaldehyderesins, polysulfones, poly(acrylonitrile-butadiene-styrene),polyurethanes, polyolefins, and polysilanes.
 28. The composite materialof claim 24, wherein the polymer comprises at least one member selectedfrom the group consisting of polyesters and polyethers.
 29. Thecomposite material of claim 24, wherein the capsules have an aspectratio of 1:1 to 1:2, and an average diameter of 10 nm to 1 mm.
 30. Thecomposite material of claim 24, wherein the capsules comprise a polymerof urea and formaldehyde, gelatin, polyurea, and polyamide.
 31. Thecomposite material of claim 13, further comprising a second plurality ofcapsules, wherein said activator is in the second plurality of capsules.32. The composite material of claim 13, wherein said correspondingactivator is a monomer.
 33. The composite material of claim 13, whereinthe polymer comprises at least one member selected from the groupconsisting of polyamides, polyesters, polycarbonates, polyethers,polyimides, phenol-formaldehyde resins, amine-formaldehyde resins,polysutfones, poly(acrylonitrile-butadiene-styrene), polyurethanes,potyolefins, and polysilanes.
 34. The composite material of claim 13,wherein the polymer comprises at least one member selected from thegroup consisting of polyesters and polyethers.
 35. The compositematerial of claim 13, wherein the capsules have an aspect ratio of 1:1to 1:2, and an average diameter of 10 nm to 1 mm.
 36. The compositematerial of claim 13, wherein the capsules comprise a polymer of ureaand formaldehyde, gelatin, polyurea, and polyamide.
 37. A method formaking the composite of claim 13, comprising: dispersing the capsulesand the corresponding activator into the polymer.
 38. A compositematerial, comprising: (i) a polymer, (ii) a polymerizer, (iii) acorresponding catalyst for the polymerizer, and (iv) a plurality ofcapsules, wherein the polymerizer is separated from the catalyst by thecapsules.
 39. The composite material of claim 38, wherein thepolymerizer comprises at least one monomer selected from the groupconsisting of cyclic olefins, lactones, lactams, acrylates, acrylicacids, alkyl acrylates, alkyl acrylic acids, styrenes, isoprene andbutadiene.
 40. The composite material of claim 38, wherein thepolymerizer comprises cyclic olefins.
 41. The composite material ofclaim 38, wherein the polymer comprises at least one member selectedfrom the group consisting of polyamides, polyesters, polycarbonates,polyethers, polyimides, phenol-formaldehyde resins, amine-formaldehyderesins, polysulfones, poly(acrylonitrile-butadiene-styrene),polyurethanes, polyolefins, and polysilanes.
 42. The composite materialof claim 38, wherein the polymer comprises at least one member selectedfrom the group consisting of polyesters and polyethers.
 43. Thecomposite material of claim 38, wherein the corresponding catalyst forthe polymerizer comprises at least one monomer selected from the groupconsisting of ROMP catalysts and cyclic ester polymerization catalysts.44. The composite material of claim 38, wherein the correspondingcatalyst for the polymerizer comprises a ROMP catalyst.
 45. Thecomposite material of claim 38, wherein the capsules have an aspectratio of 1:1 to 1:2, and an average diameter of 10 nm to 1 mm.
 46. Thecomposite material of claim 38, wherein the capsules comprise a polymerof urea and formaldehyde, gelatin, polyurea, and polyamide.
 47. Thecomposite material of claim 38, wherein the polymerizer comprises DCPD,the polymer comprises epoxy, the corresponding catalyst for thepolymerizer comprises a Grubbs catalyst, the capsules have an aspectratio of 1:1 to 1:1.5, and an average diameter of 30-300 pm, and thecapsules comprise a polymer of urea and formaldehyde.
 48. The compositematerial of claim 38, wherein the polymerizer comprises DCPD, thepolymer comprises poly(DCPD), the corresponding catalyst for thepolymerizer comprises a Grubbs catalyst, and the capsules have an aspectratio of 1:1 to 1:1.5, and an average diameter of 30-300 pm.
 49. Thecomposite material of claim 38, wherein the polymerizer comprisescaprolactone, the polymer comprises poly(caprolactone), thecorresponding catalyst for the polymerizer comprises a scandiumtriflate, and the capsules have an aspect ratio of 1:1 to 1:1.5, and anaverage diameter of 30-300 pm.